linux-kernel - Re: [PATCH v2] x86/retpoline: Add clang support

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Date:   Wed, 07 Feb 2018 22:28:38 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Guenter Roeck <linux@...ck-us.net>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Ingo Molnar <mingo@...nel.org>, gnomes@...rguk.ukuu.org.uk,
        Rik van Riel <riel@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>, thomas.lendacky@....com,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jiri Kosina <jikos@...nel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Dave Hansen <dave.hansen@...el.com>,
        Kees Cook <keescook@...gle.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...ux-foundation.org>,
        Paul Turner <pjt@...gle.com>
Subject: Re: [PATCH v2] x86/retpoline: Add clang support



On Wed, 2018-02-07 at 13:52 -0800, Guenter Roeck wrote:
> clang has its own set of compiler options for retpoline support.
> 
> Link: https://github.com/llvm-mirror/clang/commit/0d816739a82da29748caf88570affb9715e18b69
> Link: https://github.com/llvm-mirror/llvm/commit/fd5a8723ce9f2a6b250e85972ef859e4253ea95d
> Link: https://github.com/llvm-mirror/llvm/commit/59b64490fda69d29bb42cfdf7eec37bcc31ff833
> Cc: David Woodhouse <dwmw@...zon.co.uk>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: gnomes@...rguk.ukuu.org.uk
> Cc: Rik van Riel <riel@...hat.com>
> Cc: Andi Kleen <ak@...ux.intel.com>
> Cc: Josh Poimboeuf <jpoimboe@...hat.com>
> Cc: thomas.lendacky@....com
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Jiri Kosina <jikos@...nel.org>
> Cc: Andy Lutomirski <luto@...capital.net>
> Cc: Dave Hansen <dave.hansen@...el.com>
> Cc: Kees Cook <keescook@...gle.com>
> Cc: Tim Chen <tim.c.chen@...ux.intel.com>
> Cc: Greg Kroah-Hartman <gregkh@...ux-foundation.org>
> Cc: Paul Turner <pjt@...gle.com>
> Signed-off-by: Guenter Roeck <linux@...ck-us.net>
> ---
> v2: llvm has been updated to use the same thunk names as gcc.
>     Tested with:
>     clang version 7.0.0
> 	(https://git.llvm.org/git/clang.git/
> 		848874aed95a913fb45f363120500cebfe54e2ef)
> 	(https://git.llvm.org/git/llvm.git/
> 		3afd566557f3616881505db0d69f5d19bf55ae14)
>     cross-checked with gcc 7.3.0 (x86_64-linux-gcc.br_real (Buildroot
>     2018.02-rc1) 7.3.0).
> 
> Tested with 64-bit builds only; 32-bit images fail to build with clang
> with various unrelated errors and are difficult to test.
> 
> I had to change '+=' to '=' below since make otherwise sets
> RETPOLINE_CFLAGS to " ", and the subsequent ifneq would always match.
> This is also the reason for the "ifeq ($(RETPOLINE_CFLAGS),)".
> If there is another/different/better way to handle this, please let
> me know.

See
http://git.infradead.org/users/dwmw2/linux-retpoline.git/commitdiff/82a1f41600


> There are curently lots of warnings when building an image with clang.
> 
> ./include/linux/init.h:134:6: warning:
> 	unknown attribute 'indirect_branch' ignored
> 
> I was inclined to add "&& !defined(__clang__)" to the condition for the
> __noretpoline define to fix the problem, but concluded that this should
> be a separate patch unless it can be addressed in clang.

... where I fixed that too as well as providing the
__x86_indirect_thunk for 32-bit builds (which are broken for other
reasons anyway).

We do still need the %V asm constraint modifier to work in clang
though, because otherwise it still doesn't build if you have any
hypervisor guest support.

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